hold the Option key. Somehow either the boot drive won't mount and has a bad directory from crashes and errors or the path to the drive was changed and it needs to be reset in Startup Disk control panel.
Use the Lion Recovery Mode to repair the disk w/ Command + r when you boot in Disk utility and you can set the default boot drive - Startup Disk - as well.
you should have backups and be able and know how to restore and be ready at any time before it strikes.
General purpose Mac troubleshooting guide:
Mac OS X: Starting up in Safe Mode - http://docs.info.apple.com/article.html?artnum=107393
What is Safe Boot, Safe Mode? (Mac OS X) - http://support.apple.com/kb/HT1564
Isolating issues in Mac OS X
Creating a temporary user to isolate user-specific problems:
Isolating an issue by using another user account
Identifying resource hogs and other tips:
Using Activity Monitor to read System Memory and determine how much RAM is being used
Starting the computer in "safe mode":
Mac OS X: What is Safe Boot, Safe Mode?
To identify potential hardware problems:
Apple Hardware Test
General Mac maintenance:
Tips to keep your Mac in top form
Direct you to the proper forum for MacBook :
MacBook Series Forums
https://discussions.apple.com/community/notebooks?view=discussions
http://www.apple.com/support/macbookpro
Mac OS X Forum
https://discussions.apple.com/community/mac_os?view=discussions
Recovery Mode
http://support.apple.com/kb/HT4718
TimeMachine 101
https://support.apple.com/kb/HT1427
http://www.apple.com/support/timemachine
Mac OS X & Mountain Lion Community
https://discussions.apple.com/community/mac_os
https://discussions.apple.com/community/mac_os/os_x_mountain_lion?view=discussio ns
This forum deals with a desktop/tower 65lb Mac Pro
http://www.apple.com/support/macpro